There has long existed a need for a reliable device capable of objectively determining rock detachment in mines. In 1979 and 1980 alone, ninety eight fatalities and 2,344 injuries resulted from falls of roof, rib or face rock in underground coal mines. Such casualties could either be substantially reduced or eliminated through the availability of such a device.
In the past, rock detachment has been determined by tapping the rock in question with a scaling or sounding bar and listening to the resulting sounds. The lower the pitch of the detected sound, the less stable the rock. Though reasonably effective, this method proves highly subjective in application. Factors such as inexperience, inability to properly hear the rock response, the miner's health, near-by noise, obscuring dust and simple human carelessness tend to degrade one's ability to ojectively distinguish between stable and unstable rocks.
Attempts have been previously made to develop an objective method of detecting mine rock detachment. For instance, one prior art researcher determined that detached rocks vibrate at lower frequencies and for a longer time than do solid rocks when struck with a hammer or sounding bar. Others attempted to develop an instrument which would give a numerical measure of rock conditions based upon this vibrational behavior, but this effort later became abandoned.
Other prior art researchers have considered the vibrational behavior of mine rock and have confirmed that solid rocks tend to have a broad band, noise like spectrum. This response becomes greatly enhanced in the frequency range of 0 to 1,000 hertz as rock detachment increases. Again, this research failed to provide a device for ensuring an objective measure of rock detachment.
There therefore exists a need for a detached rock evaluation device that will provide an objective quantitative measure of mine rock detachment. Such a device should be relatively simple to use and durable of construction, and should be reliable and accurate in typical mine environments.